• Composites Research
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• 제34권2호
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• pp.129-135
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• 2021

본 논문에서는 필라멘트 와인딩 공정에서 발생하는 두께 방향 섬유체적비 불균일이 원통형 복합재 격자 구조의 좌굴하중에 미치는 영향을 확인하기 위해서 Vasiliev가 제안한 원통형 복합재 격자 구조 좌굴하중 이론식을 변형하여 섬유체적비에 따른 좌굴하중 저하를 확인하였다. 섬유체적비에 따라 격자 구조 리브의 각 층의 두께를 달리하였으며, 혼합법칙을 사용하여 각 층별로 물성치를 다르게 적용하였다. 구조물 크기, 두께, 섬유체적비 평균값을 달리한 유한요소모델에 대한 선형좌굴해석을 수행하였다. 최종적으로 이론식을 사용한 등가모델의 좌굴 하중 계산 결과와 유한요소해석 결과를 비교하여 두께 방향 섬유체적비 불균일이 원통형 복합재 격자 구조의 좌굴하중 저하의 원인이 될 수 있음을 확인하였다.

• Nuclear Engineering and Technology
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• 제51권4호
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• pp.977-986
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• 2019

In the thermal-hydraulic system codes, such as MARS and RELAP5/MOD3, the Savannah River Laboratory (SRL) model has been adopted as a subcooled boiling model. It, however, has been shown that the SRL model cannot take into account appropriately the effects of inlet liquid velocity and hydraulic diameter on axial void fraction development. To overcome the problems, Ha et al. (2018) proposed a modified SRL model, which is applicable to low-pressure and low-Pe conditions (P < 9.83 bar and $Pe{\leq}70,000$) only. In this work, the authors extended the modified SRL model by proposing a new net vapor generation (NVG) model and a wall evaporation model so that the new subcooled boiling model can cover a wide range of thermal-hydraulic conditions with pressures ranging from 1.1 to 69 bar, heat fluxes of $97-1186kW/m^2$, Pe of 3600 to 329,000, and hydraulic diameters of 5-25.5 mm. The new model was implemented in the MARS code and has been assessed using various subcooled boiling experimental data. The results of the new model showed better agreements with measured void fraction data, especially at low-pressure conditions.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.81-86
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• 2001

Interfacial pressure jump terms based on the physics of phasic interface and bubble dynamics are introduced into the momentum equations of the two-fluid model for bubbly flow. The pressure discontinuity across the phasic interface due to the surface tension force is expressed as the function of fluid bulk moduli and bubble radius. The consequence is that we obtain from the system of equations the real eigenvalues representing the void-fraction propagation speed and the pressure wave speed in terms of the bubble diameter. Inversely, we obtain an analytic closure relation for the radius of bubbles in the bubbly flow by using the kinematic wave speed given empirically in the literature. It is remarkable to see that the present mechanistic model using this practical bubble radius can indeed represent both the mathematical well-posedness and the physical wave speeds in the bubbly flow.

• 한국경영과학회지
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• 제19권3호
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• pp.81-91
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• 1994

This paper is to build an economic production quantity model for situations, in which, during the stockout period, a fraction .betha.(backorder ratio) of the demand is backordered and remaining fraction (1-.betha.) is lost. This paper develops an objective function representing the average annual cost of a production system by defining a time-weighted backorder cost and a lost sales penalty cost per unit lost under the assumptions of deterministic demand rate and deterministic production rate, and provides an algorithm for its optimal solution. At the extreme .betha.= 1, the presented model reduces to the Fabrycky's model with complete backorders.

• Communications for Statistical Applications and Methods
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• 제24권4호
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• pp.397-419
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• 2017

A four-parameter extended fatigue lifetime model called the odd Birnbaum-Saunders geometric distribution is proposed. This model extends the odd Birnbaum-Saunders and Birnbaum-Saunders distributions. We derive some properties of the new distribution that include expressions for the ordinary moments and generating and quantile functions. The method of maximum likelihood and a Bayesian approach are adopted to estimate the model parameters; in addition, various simulations are performed for different parameter settings and sample sizes. We propose two new models with a cure rate called the odd Birnbaum-Saunders mixture and odd Birnbaum-Saunders geometric models by assuming that the number of competing causes for the event of interest has a geometric distribution. The applicability of the new models are illustrated by means of ethylene data and melanoma data with cure fraction.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.568-573
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• 2000

In this study, experimental and numerical analyses are carried out to investigate the performance of centrifugal pump with various air admitting conditions. Experiments on pump performance under air-water two-phase flow n accomplished using a centrifugal pump with semi-open type impeller having three, five and seven blades, respectively. Also, the numerical analysis of turbulent air-water two-phase flow using finite volume method has been carried out to obtain the pressure, velocities and void fraction on the basis of a so-called bubbly flow model with the constant size and shape of cavity. The results obtained through this study show the reasonable agreements within the range of bubbly flow regime. There are promising developments concerning application of the present study for the flow in a centrifugal pump with two-phase flow conditions and efforts must be followed to improve the turbulence model and two-phase flow model for turbomachinery.

• Journal of the korean society of oceanography
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• 제39권4호
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• pp.222-233
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• 2004

Using nitrogen as the limiting nutrient, the default version of a microplankton-detritus model linked chlorophyll concentration to the autotroph nitrogen. However, phosphorus dynamics were added to simulate the results of a microcosm experiment. Using standard parameter values with a single value of microheterotroph fraction in the microplankton taken from the observed range, the best simulation successfully captured the main features of the time-courses of chlorophyll and particulate organic carbon, nitrogen and phosphorus, with root-mean-square error equivalent to 29% of particulate concentration. A standard version of microbiological model assumes complete internal cycling of nutrient elements; adding a term for ammonium and phosphate excretion by microheterotrophs did not significantly improve predictions. Relaxing the requirement for constant microheterotroph fraction resulted in an autotroph-heterotroph model AH, with dynamics resembling those of a Lotka-Volterra predator-prey system. AH fitted the microcosm data worse than did MP, justifying the suppression of Lotka-Volterra dynamics in MP. The paper concludes with a discussion of possible reasons for the success of the simple bulk dynamics of MP in simulating microplankton behaviour.

• Nuclear Engineering and Technology
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• 제51권8호
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• pp.2026-2033
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• 2019

Fluidelastic instability of a tube array is a key factor of the security of a nuclear power plant. An unsteady model of the fluidelastic instability of a tube array subjected to two-phase flow was developed to analyze the fluidelastic instability of tube bundles in two-phase flow. Based on this model, a computational program was written to calculate the eigenvalue and the critical velocity of the fluidelastic instability. The unsteady model and the program were verified by comparing with the experimental results reported previously. The influences of void fraction and the tube's material properties on the critical velocity were investigated. Numerical results showed that, with increasing the void fraction of the two-phase flow, the tube array becomes more stable. The results indicate that the critical velocities of the tube array made of stainless are much higher than those of the other two tube arrays within void fraction ranging from 20% to 80%.

• 생약학회지
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• 제51권2호
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• pp.115-121
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• 2020

Caenorhabditis elegans model system was used to investigate the antioxidant activity of methanol extract of Mallotus japonicus twigs. The ethyl acetate soluble fraction of the M. japonicus methanol extract showed the best DPPH radical scavenging activity. The ethyl acetate fraction was measured for the activity of superoxide dismutase (SOD), catalase, and oxidative stress tolerance by using C. elegans along with reactive oxygen species (ROS) level. In addition, to confirm that the regulation of the stress response gene is responsible for the increased stress tolerance of C. elegans treated by the ethyl acetate fraction, SOD-3 expression was measured using a transgenic strain. As a result, the ethyl acetate fraction increased SOD and catalase activity, and decreased ROS accumulation in a dose-dependent manner. In addition, the ethyl acetate fraction-treated CF1553 worm showed higher SOD-3::GFP intensity than the control worm.

• 생약학회지
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• 제52권4호
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• pp.251-256
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• 2021

Oat, the seeds of Avena sativa L. (Gramineae), is an important dietary staple for people in many countries. Previous studies reported that A. sativa had various pharmacological effects such as anti-inflammatory, antitumor, neurotonic, and antispasmodic activities. In this study, Caenorhabditis elegans model system was used to investigate the antioxidant activity of methanol extract of oat. The ethyl acetate soluble fraction of the oat methanol extract showed the best DPPH radical scavenging activity. The ethyl acetate fraction was measured for the activity of superoxide dismutase (SOD), catalase, and oxidative stress tolerance by using C. elegans along with reactive oxygen species (ROS) level. In addition, to confirm that the regulation of the stress response gene is responsible for the increased stress tolerance of C. elegans treated by the ethyl acetate fraction, SOD-3 expression was measured using GFP-expressing transgenic worm. As a result, the ethyl acetate fraction increased SOD and catalase activities, and decreased ROS accumulation in a dose-dependent manner. In addition, the ethyl acetate fraction-treated CF1553 worm showed higher SOD-3::GFP intensity compared to the control.